The disclosed invention is generally directed to holographic head-up display for vehicles, and is directed more particularly to a holographic head-up display that includes a guard hologram for preventing spurious reconstruction of the primary image hologram of the display by ambient light.
Head-up vehicle instrument displays have been developed to provide instrumentation information within the line of sight of the operator of a vehicle such as an automobile. One approach to head-up display systems includes a reflection hologram laminated to the inside surface of a vehicle windshield or embedded between the layers of a vehicle windshield. The reflection hologram can include operating status information recorded therein, and is intended to be reconstructed or played back by an associated light source.
While the reflection hologram of a holographic head-up display will be designed to be responsive to a playback illumination having a narrow wavelength band over a narrow angular range, it is subject to other illumination such as sunlight, street lamps or road glare. Thus, spurious reconstruction is possible as a result of illumination other than the intended playback illumination. Such spurious reconstruction is unacceptable since the display may be used for warning status information, may generate unnecessary distractions, and moreover may lead to disregard of the display.